1. Field of the Invention
This invention relates to a pressure sensor which utilizes an optical fiber as its sensing element and is highly sensitive to laterally applied pressure.
2. Background of the Invention
It is well-known that a pressure sensor can be implemented by utilizing an optical fiber that is sensitive to very small bends, commonly referred to as "micro-bends," which are imparted to the fiber. Specifically, these micro-bends react, in response to any pressure laterally applied to all or a portion of the optical fiber, by coupling substantially all the light propagating through the optical fiber from any particular propagating (core) mode to a non-propagating (clad) mode. An acousto-optic sensor which relies on this concept is described in, for example, N. Lagakos et al., "Multimode Optical Fiber Displacement Sensor", Applied Optics, Vol. 20, No. 2, Jan. 15, 1981, pages 167-168, and specifically page 167; and Hecht et al., "Fiber Optics Turns To Sensing", High Technology, July-August 1982, pages 49-56, and specifically pages 51 and 53.
Such an optical fiber is typically comprised of a optically transmissive central core, an optical cladding layer concentrically surrounding the core and having a lower index of refraction than the core, and possibly an optically opaque coating layer concentrically surrounding the cladding layer and having a higher index of refraction than the cladding layer. Whenever a micro-bend is imparted to the fiber, a portion of the light which propagates through the central core in the vicinity of each micro-bend is directed into the cladding layer. Since the coating layer has a higher index than the cladding, the coupled light is directed out of the cladding layer and is eventually absorbed by the opaque coating. This, in turn, decreases the power of the light propagating through the entire length of the optical fiber. As a result, the magnitude of any laterally applied pressure can be readily detected simply by monitoring the amplitude (or power) of the light exiting the core of the optical fiber.
Unfortunately, presently existing fiber optic pressure sensors exhibit a disappointingly low sensitivity to a laterally applied pressure. This occurs because almost all optical fibers are designed for communications--the the predominant use of optical fibers--and the design characteristics of an optical fiber suitable for low-loss optical signal transmission (communication) are generally opposite to those characteristics suitable for a pressure sensor. These latter characteristics--all of which are intentionally avoided, minimized or limited through design of a communications type fiber--include: high micro-bend losses, a large core diameter relative to the thickness of the cladding layer, a low numerical aperture of the fiber, i.e. a relatively small difference in the index of refraction between the core and the cladding layer, and a hard opaque (non-deforming) coating.
Inasmuch as few, if any, oommercially available optical fibers designed for non-communication use exist, the art turned to solutions aimed at increasing the rather low sensitivity of communication-type fibers to a laterally applied pressure. These solutions by and large all include incorporating the fiber into an external device which physically deforms the fiber to induce a substantial number of micro-bends in response to a laterally applied pressure. For example, see the apparatus described in the above-cited Lagakos article and in U.S. Pat. No. 4,342,907 (issued to P. B. Macedo et al on Aug. 3, 1982). Unfortunately, the use of these external devices adds several drawbacks, noteably increased bulk, cost and mechanical complexity, to an optical fiber based pressure sensor.